This is a thought that has bothered me a while. Let me try explaining:
Assuming I made 100c (SomeCurrency) and of that I pay say 20% as income tax. With the 80c I buy something that costs 10c but pay sales tax of say 12% on it (so total tax is 20c+1.2c). But of the 8.8c the product actually cost me, the company pays an employee say 1c of which he pays 20% and so on (so total tax increases by 0.2c). Assuming we spend all 100c every month, how much of our monthly income would go towards tax?

You can't count both taxes, the one related to your income and the one related to whoever your pay income.

Also this doesn't look like a math question (calculating how much you spend on taxes knowing all the taxes and how you spent your money is quite simple). It is more related to politics/economy and it changes country to country (I'm from Argentina and we have some higher taxes here, for example). Also, taxes are too complex in the sense that there are really many different ones and are applied in different situations. I think there are estimatives for different countries, but you will never be sure how much of your income went to the State.

Assuming we forget all other tax except for income tax. It is a flat 25% for everyone who makes any income. If I was to buy a keyboard, and paid x for it. This x pays for the people who work for the company that makes the keyboard, who pay tax on it. But the money they spend getting the plastic pays the plastic dealers who pays tax, and then the guy on the oil rig makes money from the plastic guy, and pays tax there, and the guy who grows the coffee that the oil rig guy is drinking pays tax. So in effect, your buying a keyboard is effectively paying the coffee grower's taxes, probably very minuscule, but still there. Right?